This module extends the CSS sizing properties with keywords that represent content-based "intrinsic" sizes and context-based "extrinsic" sizes, allowing CSS to more easily describe boxes that fit their content or fit into a particular layout context.CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.
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This section is not normative.
CSS layout has several different concepts of automatic sizing that are used in various layout calculations. This section defines some more precise terminology to help connect the layout behaviors of this spec to the calculations used in other modules, and some new keywords for the width and height properties to allow authors to assign elements the dimensions resulting from these size calculations.
This module extends the width, height, min-width, min-height, max-width, max-height, and column-width features defined in [CSS21] chapter 10 and in [CSS3COL]
This specification follows the CSS property definition conventions from [CSS21]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS modules may expand the definitions of these value types: for example [CSS3COLOR], when combined with this module, expands the definition of the <color> value type as used in this specification.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.
Note: This is the formula used to calculate the auto widths of non-replaced blocks in normal flow in CSS2.1§10.3.3.
Note: This is called the “available width” in CSS2.1§10.3.5 and computed by the rules in CSS2.1§10.3.3.
Note: This is called the “preferred width” in CSS2.1§10.3.5 and the “maximum cell width” in CSS2.1§126.96.36.199.
Note: This is called the “preferred minimum width” in CSS2.1§10.3.5 and the “minimum content width” in CSS2.1§188.8.131.52.
min(preferred inline-size, max(min-content inline-size, fill-available inline-size)). Otherwise, equal to the max-content inline-size.
Note: This is called the “shrink-to-fit” width in CSS2.1§10.3.5 and CSS Multi-column Layout § 3.4.
Or should this be the minimum between allowed break points? It might make sense in multi-col contexts to have min-content and max-content block-sizes be different, even if they are the same elsewhere.
Same issue as min-content block-size.
|Name:||width, min-width, max-width, height, min-height, max-height|
|New values:||fill | max-content | min-content | fit-content|
There are four types of automatically-determined sizes in CSS (which are represented in the width and height properties by the keywords defined above):
Does this value work? Is it needed? Feedback from dbaron
Right now all of these except fill mean the same thing for block-sizes. This may or may not be ideal.
If the inline-size is auto, we could have min-content block-size imply a max-content inline-size, and vice versa.
Note that percentages resolved against the intrinsic sizes (max-content, min-content, fit-content, repudiate-floats) will compute to auto, as defined by CSS 2.1. [CSS21]
|New values:||fill | max-content | min-content | fit-content|
When used as values for column-width, the new keywords specify the optimal column width:
min(max-content inline-size, max(min-content inline-size, fill-available inline-size)).
Intrinsic sizing determines sizes based on the contents of an element, without regard for its context.
For replaced elements, the min-content size and max-content size are equivalent and correspond to the appropriate dimension of the concrete object size returned by the default sizing algorithm [CSS3-IMAGES] of the element, calculated with an unconstrained specified size.
The min-content inline-size of an inline box is the length of the largest unbreakable sequence of inline content. The min-content inline-size contribution of an inline box is its min-content inline-size, plus any inline-axis margin, border, and padding adjacent to that sequence.
The max-content inline-size of an inline box is the length of the largest sequence of inline content on a single line when only forced line breaks are taken. The max-content inline-size contribution of an inline box is its max-content inline-size, plus any inline-axis margin, border, and padding adjacent to that sequence.
The min-content block-size, max-content block-size, min-content block-size contribution, and max-content block-size contribution of an inline box are the distance from the head edge of the first line box to the foot edge of the last line box on which the inline appears.
The min-content inline-size of a block container box is the largest min-content inline-size contribution of its in-flow or floated children.
The max-content inline-size of a block container box is the inline-size of the box after layout, if all children are sized under a max-content constraint.
If the computed inline-size of a block-level box is min-content, max-content, or a definite size, its min-content inline-size contribution is that size plus any inline-axis margin, border, and padding. Otherwise, if the computed inline-size of the block is fit-content, auto, or fill, its min-content inline-size contribution is its min-content inline-size plus any inline-axis margin, border, and padding.
If the computed inline-size of a block-level box is min-content, max-content, or a definite size, its max-content inline-size contribution is that size plus any inline-axis margin, border, and padding. Otherwise, if the computed inline-size of the block is fit-content, auto, or fill, its max-content inline-size contribution is its max-content inline-size plus any inline-axis margin, border, and padding.
The min-content block-size and max-content block-size of a block container box is the content block-size as defined (for horizontal writing modes) in CSS2.1§10.6.3 and CSS2.1§17.5.3 for elements with height: auto, and analogously for vertical writing modes.
The min-content block-size contribution and max-content block-size contribution of a block-level box is the block-size of the block after layout, plus any block-axis margin, border, and padding.
The min-content inline-size of a multi-column element with a computed column-width not auto is the smaller of its column-width and the largest min-content inline-size contribution of its contents.
The min-content inline-size of a multi-column element with a computed column-width of auto is the largest min-content inline-size contribution of its contents multiplied by its column-count (treating auto as 1).
The max-content inline-size of a multi-column element with unrestrained column heights and a computed column-count not auto is its column-count multiplied by the larger of its column-width (treating auto as 0) and the largest max-content inline-size contribution of its contents.
The max-content inline-size of a multi-column element with unrestrained column heights and a computed column-count of auto is its column-width multiplied by the number of columns obtained by taking all allowed column breaks [CSS3-BREAK].
The max-content inline-size of a multi-column element with restrained-height columns (i.e. a specified height or max-height, or whichever properties map to the block-size of the element) is the inline-size that would exactly include all of its columns. It may be approximated by:
or by some more accurate method.
This approximation can result in some slack, but avoids overflow in the most common cases, where the balanced height of the columns above spanning elements are approximately equal.
In the common case of no column-spanning elements, this approximation collapses to simply doing a layout, and measuring the resulting columns.
The preferred inline-size of a multi-column element with non-auto column-width and column-count is the product of the column-width and column-count, plus the appropriate amount of column gaps. In all other cases, it is the max-content inline-size.
Extrinsic sizing determines sizes based on the context of an element, without regard for its contents.
The inner fill-available inline-size of a box is…
max(min-content inline-size|0, min(max-content inline-size|infinity, inline-size|fill-available inline-size))where the sizes are inner inline-sizes of the element establishing the box’s containing block, and where the first value is used if it is definite and the second value otherwise.
…less the box’s inline-axis margins (after any margin collapsing, and treating auto margins as zero), borders, and padding.
The fill-available block-size of a box is defined analogously, but in the other dimension.
This definition might end up skipping further up the ancestor chain than we’d like in some cases. Example. Maybe it should stop at each formatting root, or something similar?
Special thanks go to Aaron Gustafson, L. David Baron for their contributions to this module.
Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
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Note, this is an informative note.
Conformance to this specification is defined for three conformance classes:
A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.
A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.
To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.
Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
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Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the email@example.com mailing list.
No properties defined.